1. Field of the Invention
The present invention relates to a vehicle state estimation method and a vehicular auxiliary brake control apparatus which uses the estimation method. In particular, the invention relates to a method for estimating the vehicle state of a commercial vehicle capable of carrying loads or passengers such as a truck or a bus and to a vehicular auxiliary brake control apparatus using that estimation method.
2. Description of the Related Art
In commercial vehicles, the load amount greatly varies from an empty state to a loaded state even though the engine is small for the vehicle's own weight. When such a commercial vehicle is heavily loaded with goods, passengers, or the like and its weight (total weight) is increased accordingly, sufficient braking performance is not attained on a downhill slope or the like because the engine braking becomes less effective. This necessitates control of the driving force or the braking force in consideration of the vehicle weight (total weight)
Conventionally, in estimating the vehicle state, a comprehensive parameter of a vehicle load as represented by vehicle load torque TI that is the sum of aerodynamic drag torque Ta, rolling resistance torque Tr, and surface gradient resistance torque T.theta. is estimated based on the relationship between the driving torque and the acceleration by using a vehicle weight sensor, a torque sensor, or the like. However, this method causes an estimation error when the road surface has a gradient as in the case of running on a sloping road. Further, in this method, the vehicle weight and the road surface gradient cannot be separated properly from the vehicle load torque.
In general, in automobiles, the road surface gradient can be estimated relatively easily with an assumption that the vehicle weight is constant, because no large variation occurs in vehicle weight. On the other hand, in vehicles such as commercial ones in which the vehicle weight varies greatly, it cannot be judged whether a vehicle load variation is due to a variation in road gradient or a variation in vehicle weight. This necessitates estimation of the road surface gradient, and hence the estimation method for automobiles cannot be used for commercial vehicles.